A controller for use of an internal combustion engine capable of varying an overlapping term in which both intake and exhaust valves open is conventionally used. For example, Japanese Patent Public Disclosure (Kokai) No. 2-119641 discloses a controller having a mechanical supercharger driven by the engine for supercharging intake air and varying the overlapping term in accordance with driving condition of the engine so that the overlapping term is set to be longer as engine speed is higher. It should be noted that the term "an overlapping term" will be used hereinbelow to mean the term in which both intake and exhaust valves open.
Furthermore, Japanese Patent Public Disclosure (Kokai) No. 63-239312 discloses a controller which controls an engine having a mechanical supercharger so that "the degree of delay" is set to be smaller as the overlapping term is longer, whereby knocking of the engine being avoided. The term "the degree of delay" means the degree indicating how later intake valves close than usual. Hereinafter, the term "the degree of delay" will be used to have such a meaning.
An engine having a supercharger including a turbocharger can decrease density of gas residual in a combustion chamber due to fresh intake air, and consequently decrease a temperature of intake air to decrease a frequency of knocking when the overlapping term is set to be long on the condition that supercharged pressure is higher than pressure of exhaust gas.
When intake valves are closed later than usual, a compression ratio can be substantially decreased. In addition, intake air is compressed by a supercharger, cooled by an intercooler and then provided to the engine, whereby an amount of intake air filled by means of a supercharger can be increased while a compression ratio can be decreased to decrease a temperature of compressed intake air and anti-knocking property can be enhanced.
However, there arise problems when the overlapping term is set to be long and intake valves are made to close later than usual for the purpose of enhancing anti-knocking property. That is, when the overlapping term is set to be long, the overlapping term tends to be longer at low engine speed than at high engine speed, so that so-called "blow-by" is likely to occur. The term "blow-by" means that a part of fuel provided into intake ports passes through a combustion chamber without burning to exhaust ports. In addition, when a piston ascends from BDC, a part of fuel present in a combustion chamber returns through intake valves, which are opening then, to the intake ports. This part of fuel passes to exhaust ports through a combustion chamber without burning when intake valves open next and overlap exhaust valves in opening. Thus, "blow-by" of fuel occurs.
On the other hand, when intake valves are closed later than usual, a load to a supercharger is increased in order to increase supercharging pressure. This arises a problem that a temperature of intake air discharged from a supercharger increases and the temperature tends to reach a critical point in terms of thermal expansion of compression portions in a supercharger.
The present invention is made to solve one or more of aforementioned problems.